Recent comments from SciRate

Hi Mikhail,

You are basically saying that the exact overlap between the ideal and non-ideal state decays exponentially in N. I agree with you on this but you are merely attacking a straw man here. Exponentially small overlap does not imply that you cannot do fault-tolerant quantum computation. No

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Dear Barbara,
I agree, and this is also my point: " **it cannot be said that the theory of quantum error correction and fault-tolerance guarantees that robust quantum computers will ever be built**.
As well as that **this is given by "the physics"**.

In particular the **quantum physics** which

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Dear Elizabeth, I fully agree with what you are saying.

Except that faulty gates are not the only source of errors. There are also unwanted interactions within the system of qubits, as well as between this system and the environment, and also because the initial state cannot be exactly |000...>,

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> It's not a complete analogy to the authentication game that Sandu has
> the parties play

Here's a closer classical analogy: Alice prepares an ensemble of classical bits. Each bit is random (determined by a coin toss) and, for some fixed partitioning of the bits into pairs, Alice writes down i

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Victory Omole, The Nobel prize was given "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems."

Here "Individual quantum systems*" means individual atoms and individual photons, definitely NOT many-body quantum systems, as you apparently

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My comment was not so much in reply to Dyakonov's paper (i.e. a corroboration or refutation of his points) but rather a general contribution to the discussion.

I think we sometimes tend to respond to critics by rewriting their criticism into something more reasonable. Are correlations in the power spectrum of noise something that may be influenced by our gates and may await more experimental data and more theoretical insight? Yes, probably. Is this what

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Naturally, it cannot be said that the theory of quantum error correction and fault-tolerance guarantees that robust quantum computers will ever be built. One particular challenge is the ability to turn gates off and on: in almost all implementations this works by meeting resonance conditions which a

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Even without studying quantum information theory, there is an immediate way to see that quantum computation is more robust than classical analog computation. This argument is based on linearity. Consider a sequence of unitary operators $U_1 ,...,U_T$. For concreteness, each $U_i$ acts on two qub

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>My previous experience tells me that it is never possible to control a many-body quantum (or even classical) system on a microscopic level.

If your previous experience tells you this, then i encourage you to gain new experience because experimentalists have been controlling many-body quantum sys

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Dear Steve,

Thank you very much for your invitation and your friendly attitude! However it seems to me that we live in parallel worlds: the world of quantum information theory, which has a rather poor experimental support, and the world of physics, where the relationship between theory and experi

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Michel, you should come to a quantum information conference!

You may feel like your critiques have fallen on deaf ears, but that is not true. If we don't pay explicit attention it is because, as Aram and others have pointed out above, we feel that we have already addressed your concerns. I know

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OK, I have not studied these papers yet.
Let us return to this subject in 10 years when hopefully someone will manage to factor 15 by Shor (full Shor's algorithm, please, no cheating with the "compiled version".

This is awesome!

One more thing. The fidelity between the actual and ideal states will be exponentially small but this is not a barrier to FTQC. The same occurs with classical memory.

The classical analogue of the fidelity expression you quote is: "What are the odds that all the spins in your hard drive are poi

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That is some evidence it can be efficiently scaled up. You would have said the same thing if those experiments were not done at all; but the number and quality of qubits experimentalists encode fault tolerantly seem to be increasing with time and effort and there is no evidence it's going to stop.

This objection is a bit of a moving target. First there is the proposal that there is some reason that quantum computers cannot work _in principle_. Then when people respond by saying that this reason is (a) vague and unspecified and (b) contrary to accepted principles of locality in physics, the

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Of course there will be some undesired extra terms in the Hamiltonian, which we can call V. However, V is not a completely arbitrary $2^n$-dimensional matrix. We can expand it as
$$ V = V_0 + V_1 + V_2 + \cdots + V_n .$$
where $V_j$ contains only tensor products of $j$ non-identity Pauli matrice

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So far there is no evidence that this can be efficiently scaled up

The good news is that some basic principles of quantum error-correction have already been demonstrated on existing quantum computing hardware, e.g. see https://arxiv.org/pdf/1806.02359.pdf and references therein.

I would like to see an experimental realization of these ideas with a large enough number of qubits, say 10 -20

In quantum error correction we deal with this problem by actively performing error-detecting measurement and correcting the error. The measurement will be faulty in reality, but this can be dealt with by repeating the measurement; see Section IV of https://arxiv.org/abs/quant-ph/0110143 for example.

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No doubt that the Schroedinger equation describes the evolution of the wavefunction, hence of its 2^N amplitudes. The problem is that in reality the Hamiltonian, and hence the evolution of the quantum amplitudes, can never be exactly what you would like it to be: H=H_0 +V. Also, the initial conditio

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The wavevector is determined exactly as the solution to the equation
$$ \frac{d}{dt}|\psi(t)\rangle = -iH(t) |\psi(t)\rangle.$$
While the state has $2^n$ dimensions, those do not show up as experimentalist-tunable parameters in this equation. How many tunable parameters are there? Well, those s

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"The only place where 2^N parameters appear is in the wavefunction, not in the Hamiltonian or Lagrangian".

- Certainly. As explicitly stated, I am talking about the wavefunction, which describes the **state** of the quantum system at a given time. You seem to agree that this state is generally d

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This argument lacks detail so it is hard to clearly refute, however one point is clearly wrong. A quantum computer, or quantum mechanics experiment, with $N$ qubits can be described by $O(N)$ or at most $O(N^2)$ parameters. This is because physical Hamiltonians are usually described by 2-body inter

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Dear Blake,

The packing (i.e. the title) is catchy and eye-candy indeed and you are right to accuse the autors of over-selling the content.
At the same time, to be fair, we need to acknowledge their reservation: ,,accepting the photons’ status as observers'' is condicio sine qua non for the resu

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I don't think one can call an experiment an implementation of the Wigner's friend scenario if it's an experiment on photons. The essence of the thought-experiment is that quantum weirdness should apply to a sufficiently careful preparation of *an entire living, thinking observer.* If the "friends" a

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This paper draws a connection between "antidistinguishability" and Symmetric Informationally Complete measurements. One example of such a connection was already known, and it's a bit of a funny story. If you go back to Caves, Fuchs and Schack's "Conditions for compatibility of quantum state assignme

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Yes, I was aware of Rovelli's paper and my aim was to be loosely consistent with it, although the papers differ in that he is concerned with the entropic arrow of time, whereas I am mainly concerned with the causal arrow. It is a deep question what the connection is between the two, though the sugge

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Yes, well spotted! I made an error of omission in the definition of `layered'. It should include the proviso that "every path from an ancestor of X to a descendant of X is intercepted by a node in the layer containing X". That is what I meant, and it is assumed in order to carry out the proofs in th

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We welcome any and all citation requests, even those having to do with pop culture. Thanks for sharing :)

"Thauma" is Greek for "wonder or marvel":

https://en.wiktionary.org/wiki/%CE%B8%CE%B1%CF%8D%CE%BC%CE%B1

which preceded Pratchett by several millennia :)

We indicated this Greek origi

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Love the idea! I'm surprised to see no citation of Terry Pratchett, though. https://wiki.lspace.org/mediawiki/Thaum

Makes sense. Thank you Wojciech.

Dear Marcel,

This condition is just for the ,,All'' face of their Janus-like theorem, and would $ \Omega $ be indeed inaccessible, we would experience just the ,,Nothing''-face.

That is how I understand that.

BR,
Wojciech

I struggle a bit with section 3.4:
"... if the experimenters are given access to an incompressible number (such as Ω) ...".

Isn't the point that exactly this is not possible, because numbers like Ω are not computable?

Hi Tomoyuki,

Thank you for bringing your work to our attention. This paper is a follow-up to [arXiv:1804.10368v2][1], which were together submitted to a journal in December. We will be happy to cite your concurrent work, and are excited to hear that other people are thinking along the same line

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The result in Theorem 1 of this paper was already shown in Theorem 6 of our paper
https://arxiv.org/pdf/1901.01637.pdf

You're right, Aaronson's result applies to integer matrices. I should have linked to [Valiant's original paper][1] which provides a reduction between the case of integer matrices and 0-1 matrices.

[1]: https://doi.org/10.1016/0304-3975%2879%2990044-6

While it is #P hard to compute the permanent of 0-1 matrices, the paper you linked doesn't show this fact. The linear optical proof of Aaronson applies to matrices with entries complicated enough that they can represent universal quantum computation when taken as the beam-splitting unit in a linear

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Doesn't this contradict the fact [computing the permanent is #P-hard][1]? In particular, it is also NP-hard, so it should not have a polynomial-time algorithm.

[1]: https://doi.org/10.1098/rspa.2011.0232

Casual readers of this article, especially those in the target audience of students with an engineering background, might be misled by the way that the term "defect" is being used here in the title and abstract. In an engineering or manufacturing context, a defect is an imperfection that is to be av

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Your work - being a very nice technical result in the framework of causal modeling - has also a familiar ring to the ideas of Carlo Rovelli on [perspectival time's arrow][1].
A little different genres but the same main theme: the asymmetries of time are *produced* by the observer, rather than being

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"Result 1" of this article is "Tomography...is possible...if and only if G is a *layered* DAG.
Nikolov and Tarassov [Discrete Applied Mathematics 154 (2006) 848-860] state (in their first paragraph) that *any* DAG can be layered.

A big hit from 1997 ([https://doi.org/10.1038/37539][1]) with more than 5000 citations. Great to see it also on arXiv.

[1]: https://doi.org/10.1038/37539

Thank you for your comment.

One conclusion which can be drawn from our work, and which is also found in other work such as the one you reference, is of the “overcompleteness” of the axioms of quantum theory. By this I mean that one can remove one of the axioms of the theory and then recover it fr

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Thanks for the interesting idea! One quick question is what the results of two same experiments that start and end at the same time? Will these results be exactly the same?

We studied this type of diffusion-parameter-estimation problem in

(1) Shilin Ng, Shan Zheng Ang, Trevor A. Wheatley, Hidehiro Yonezawa, Akira Furusawa, Elanor H. Huntington, and Mankei Tsang, "Spectrum analysis with quantum dynamical systems," Physical Review A 93, 042121 (2016); http://dx.doi.org/

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Very interesting. I should read your paper more carefully, but it sounds like this (pretty old) paper by Vladimir Buzek, Nicolas Gisin and myself (which was based on even older work by Nicolas Gisin) is in some sense complementary: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.87.170405

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In response to the government shutdown I have put the zoo up at:

http://quantumalgorithmzoo.org/

It remains 6 months out of date, but I hope to update soon....